Reactor system for ether production

ABSTRACT

Isopentene, or isoamylene, conversion to methyl tert-amyl ether can be substantially improved while high conversion of isobutylene to methyl tert-butyl ether can be maintained by carrying out the overall etherification process with alkanol in a staged manner, wherein the first stage is methanol etherification of a C 5  +, or C 5 , hydrocarbon feedstream rich in isoamylene and the second stage is etherification to produce MTBE and additional TAME from a C 4  +, or C 4 , feedstream. Unreacted methanol and hydrocarbons from the first etherification are uniquely separated by fractionation from the TAME product by using the second stage C 4  + feedstream as a reflux stream to the fractionator and passed to the second etherification zone. Products from the second etherification zone are separated by distillation to produce MTBE, TAME and C 5  +, or C 5 , hydrocarbons as a bottom stream.

REFERENCE TO COPENDING APPLICATION

This application is a continuation-in-part of U.S. Pat. application Ser.No. 07/427,221 file Oct. 24, 1980, now U.S. Pat. No. 4,988,36incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to the production of high octane tertiary alkylethers and gasoline. In particular, the invention relates to theproduction of methyl tertiary butyl ether (MTBE) and methyl tertiaryamyl ether (TAME) utilizing a reactor system which significantlyimproves the conversion of isoamylene to TAME and produces high octanegasoline.

It is well known that isobutylene may be reacted with methanol over anacidic catalyst to provide methyl tertiary butyl ether (MTBE) andisoamylenes may be reacted with methanol over an acidic catalyst toproduce tertiary amyl methyl ether (TAME). The reaction is a usefulpreparation for these valuable gasoline octane enhancers and is typicalof the reaction of the addition of lower alkanol to the more reactivetertiary alkenes, or iso-olefins, of the type R₂ C═CH₂ or R₂ C═CHR undermild conditions to form the corresponding tertiary alkyl ethers. Thefeedstock for the etherification reaction may be taken from a variety ofrefinery process streams such as the unsaturated gas plant of afluidized bed catalytic cracking operation containing mixed lightolefins, preferably rich in isobutylene and isopentenes or isoamylene.

Generally, it is known that asymetrical C₅ -C₇ alkyl tertiary alkylethers are particularly useful as octane improvers for liquid fuels,especially gasoline. MTBE, ethyl t-butyl ether (ETBE), isopropyl t-butylether (IPTBE) and TAME are known to be high octane ethers. The articleby J.D. Chase, et al., Oil and Gas Journal, Apr. 9, 1979, discusses theadvantages one can achieve by using such materials to enhance gasolineoctane. The octane blending number of MTBE when 10% is added to a basefuel (R+O=91) is about 120. For a fuel with a low motor rating (M+O=83)octane, the blending value of MTBE at the 10% level is about 103. On theother hand, for an (R+O) of 95 octane fuel, the blending value of 10%MTBE is about 114. Increasing demand for high octane gasolines blendedwith high octane ethers as octane boosters and supplementary fuels hascreated a significant demand for these ethers, especially MTBE and TAME.

In these etherification processes, a problem of major importance is theseparation of methanol from the etherification reaction product due tothe proclivity of methanol to form a very dilute azeotropic mixture withhydrocarbons and the strong solubility of methanol in both water andhydrocarbons. Due largely to these factors, the cost associated withmethanol separation and recycling in the etherification reactionrepresents approximately 30% of the cost of the total etherificationprocess. While it would be useful from an equilibrium standpoint to uselarge excesses of methanol in etherification, subsequent separationproblems have limited that process improvement. Currently, preparationof MTBE and TAME is carried out using C₄ + hydrocarbon feedstock wheremethanol is present in the etherification step in about less than athree weight percent excess based on iso-olefins in the feed. This iseffective in converting over ninety percent of isobutylene to MTBE, butisoamylene conversion is limited to about sixty-five percent under theseconditions. Attempts to improve the conversion of isoamylene to TAME bymanipulating the chemical equilibria with large excesses of methanol,while maintaining high conversion of isobutylene to MTBE have provendisappointing, incurring heavy economic burdens on separation of theproduct.

Representative teachings in the prior art directed to the effort toimprove the iso-olefin etherificaton process include U.S. Pat. No.4,647,703 to Torck et al. which describes a multi-stage etherificationprocess wherein effluent from the first stage is passed to afractionator, a bottoms product containing ethers is withdrawn, and atop product containing unreacted light olefins and alcohol is passed toa second stage etherification reactor.

In U.S. Pat. No. 4,554,386 to Groeneveld et al. an iso-olefinetherification process is disclosed wherein multiple reactors areemployed. An MTBE separation column is positioned after the firstreactor.

In U.S. Pat. No. 4,324,924 to Torck et al. a multi-stage process isdisclosed for preparing MTBE wherein effluent from the first stage isfractionated and the overhead is passed to a second stage forprocessing.

It is an object of the instant invention to provide a process for theproduction of MTBE and TAME that includes high conversion of isoamyleneto TAME.

SUMMARY OF THE INVENTION

It has been discovered that the conversion of isopentene, or isoamylene,to methyl tert-amyl ether can be substantially improved while thetypically high conversion of isobutylene to methyl tert-butyl ether canbe maintained by carrying out the overall etherification process withalkanol such as methanol in a staged manner, wherein the first stage ismethanol etherification of a C₅ +, or C₅, hydrocarbon feedstream rich inisoamylene and the second stage is etherification to produce MTBE andadditional TAME from a C₄ +, or C₄, feedstream. In the first stageetherification, the methanol concentration or feed rate is maintained atan amount sufficient to assure that dowmstream fractionation of thereaction effluent produces low methanol carry-over and a TAME productwith low methanol content. Unreacted methanol and hydrocarbons from thefirst etherification stage are uniquely separated by fractionation fromthe TAME product by using the second stage C₄ + feedstream as a refluxstream to the fractionator. The methanol-free added reflux streamprovides additional hydrocarbon needed to separate methanol and enhancemethanol flowrate in the fractionator overhead stream. With the additionof fresh methanol, as may be required, these streams are passed to thesecond etherification zone. Products from the second etherification zoneare separated by distillation, preferably in a debutanizer, to produceMTBE, TAME and C₅ +, or C₅, hydrocarbons as a bottom stream.

More particularly, the invention is a novel reactor system for theproduction of alkyl tertiary alkyl ethers, comprising in combination:first reactor means for etherification of C₅ + iso-olefins; fractionatormeans operatively connected to said first reactor means to separateeffluent therefrom as overhead and bottom streams, said fractionatorcontaining means for receiving C₄ + hydrocarbon feedstream as refluxstream in an upper portion; second reactor means for etherification ofC₄ + iso-olefins operatively connected to receive said overhead streamfrom said fractionator; and debutanizer means operatively connected tosaid second reactor for separating effluent therefrom.

DESCRIPTION OF THE FIGURE

The figure is a schematic diagram of the process of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Typical hydrocarbon feedstock materials for etherification reactionsinclude olefinic streams, such as FCC light cracked gas containingbutene isomers, often in mixture with substantial amounts of propene,propane, n-butane and isobutane. The C₄ components usually contain amajor amount of unsaturated compounds, such as 10-20% isobutylene,30-50% linear butenes, and small amounts of butadiene. Also, C₄ +heavier olefinic hydrocarbon streams may be used, particularly mixturesof isobutylene and isoamylene. These aliphatic streams are produced in avariety of petroleum refinery operations such as catalytic cracking ofgas oil or the like. C₅ and C₅ + hydrocarbon streams containingisoamylene may be obtained from similar sources by debutanization.

The reaction of methanol with isobutylene and isoamylenes at moderateconditions with a resin catalyst is known technology, as provided by R.W. Reynolds, et al., The Oil and Gas Journal, Jun. 16, 1975, and S.Pecci and T. Floris, Hydrocarbon Processing, Dec. 1977. An articleentitled "MTBE and TAME - A Good Octane Boosting Combo," by J.D. Chase,et al., The Oil and Gas Journal, Apr. 9, 1979, pages 149-152, discussesthe technology. A preferred catalyst is a sulfonated polystyrene resinor polymeric sulfonic acid exchange resin such as Amberlyst 15. Otheracidic catalysts may be used. Acidic zeolites, such as ZSM-5 and zeoliteBeta, are particularly useful catalysts.

Processes for producing and recovering MTBE and other methyl tertiaryalkyl ethers from C₄ -C₇ iso-olefins are known to those skilled in theart, such as disclosed in U.S. Pat. Nos. 4,544,776 (Osterburg, et al.)and 4,603,225 (Colaianne et al.). In the prior art various suitableextraction and distillation techniques are known for recovering etherand hydrocarbon streams from etherification effluent.

In the present invention methanol is the preferred lower or lightalkanol. However, other alkanols may be used. Lower alkyl or alkanol inthe present invention refers to C₁ -C₄ alkyl derived from etherificationusing methanol, ethanol, 1-propanol, isopropanol, 2-butanol and1-butanol. Tertiary alkyl refers to C₄ -C₅ tertiary alkyl groups derivedfrom the etherification of iso-olefins such as isobutene and isoamylene.

Methanol may be readily obtained from coal by gasification to synthesisgas and conversion of the synthesis gas to methanol by well-establishedindustrial processes. As an alternative, the methanol may be obtainedfrom natural gas by other conventional processes, such as steamreforming or partial oxidation to make the intermediate syngas. Crudemethanol from such processes usually contains a significant amount ofwater, usually in the range of 4 to 20 wt%.

Isobutylene and isoamylene etherification conditions are known in theart and, in the instant invention, comprise mild conditions of lowtemperature and high liquid hourly space velocity (LHSV). Etherificationtemperature can range from 20° C. to 150° C. and preferably between 60°and 125° C.; more preferably between 50°-70° C.

The reactor system of the present invention produces alkyl tertiaryalkyl ethers include methyl tertiary butyl ether, methyl tertiary amylether, ethyl tertiary butyl ether, ethyl tertiary amyl ether, isopropyltertiary amyl ether and isopropyl tertiary butyl ether. In the preferredembodiments of this invention to produce high octane ethers, methanol isreacted in contact with acidic catalyst with C₅ or C₅ + olefinichydrocarbon feedstock rich in isopentene to produce methyl tertiary amylether. Preferably, at least 50% more methanol leaves the first stagereactor than in a conventional TAME process which separates methanolfrom TAME using a fractionation tower. In the process unreacted methanolin the first stage effluent comprises between 0.1 and 100 weightpercent, based on unreacted isoamylene therein. Preferably, unreactedmethanol in the effluent comprises about 7-70 weight percent, based onisoamylene therein. As a consequence of carrying out the etherificationin the absence of more reactive iso-olefins such as isobutylene andforcing the equilibrium toward the formation of ether by enhancingmethanol flowrate or concentration, the conversion of isoamylene issubstantially improved and production of TAME increased. However, therelatively high flow of unreacted methanol conventionally would end upin the TAME product as a result of the formation of a dilute, lowerboiling azeotrope of methanol and C₅ hydrocarbon during fractionation.Typically, this would present problems of an aqueous phase formation ingasolines containing the product.

The present invention avoids the foregoing separation problems ofrelatively high flow of unreacted methanol by providing an olefinic C₄or C₄ + feedstream to the TAME fractionator which is placed in aninterstage configuration upstream of an MTBE/TAME etherificationreactor. Preferably, the hydrocarbon stream is added in an upper portionof the fractionator as a tower reflux at a tower top temperature ofabout 38° C. and pressure of less than about 100 psig. With the addedhydrocarbon reflux feedstream the formation of azeotrope and separationof unreacted methanol is readily accomplished. The tower overheadcontaining methanol and C₄ + hydrocarbons including isobutylene andisoamylene is utilized as the feedstream to a second etherification zoneto form MTBE and additional TAME. Fresh methanol, as required, may alsobe fed to the second etherification zone. The effluent from the secondzone is separated, preferably in a debutanizer, to provide C₅ + gasolinerich in MTBE and TAME as a bottom stream and an overhead streamcontaining unreacted methanol and C₄ -hydrocarbons.

Referring to the Figure, a schematic flow diagram depicts the presentinvention. The diagram shows the following principal components for thenovel reactor system: a TAME etherification reactor zone A, preferablycontaining an acidic zeolite catalyst; a reactor zone B, also containingacidic catalyst such as Amberlyst 15 from Rohm and Haas foretherification to produce MTBE and TAME; and fractionators C and D forthe separation of the products from the etherification zones A and B,respectively. The reactor zones A and B may each contain severalreactors connected in series through heat exchangers in order to controlthe etherification reaction exotherm. Generally, these reactor zonesoperate at temperatures between about 30° C. and 150° C. at moderatepressure.

A C₅ or c₅ + hydrocarbon feedstream 105 which is rich in isoamylene ispassed to the etherification zone A together with a methanol feedstream110. The effluent 112 from reactor A is passed to heat exchanger unit114 with the bottoms stream 115 from the fractionator C. The heatexchanged stream is then passed via line 116 and water via line 117 toan extraction unit 118 wherein water-soluble reaction components arepartially removed prior to passing via stream 119 to a mid-portion ofthat fractionator. The water wash means 118 removes sufficient methanolfrom the first reactor effluent stream to prevent azeotrope formation inthe following fractionation. The 112 effluent comprises TAME, unreactedmethanol and C₅ or C₅ + hydrocarbons. A C₄ or C₄ + stream 118 containingisobutylene is introduced into a top portion of the fractionator C,forming at least a component of the fractionator reflux stream 120comprising C₅ -hydrocarbons and methanol. The quantity of the addedreflux stream is sufficient, when combined with C₅ -hydrocarbonscontained in the 116 effluent, to assure the separation of unreactedmethanol in the 112 stream into the overhead stream 122. The 115 bottomstream from the fractionator comprises C₆ + products containing TAME.When the 105 feedstream is C₅ +, higher gasoline bottom stream. Theoverhead stream 122, now containing C₄ + hydrocarbons and methanol ispassed to reactor B, preferably in conjunction with added methanolfeedstream 124 to make up a slight excess, about 1 weight percent, ofmethanol in the reactor compared to iso-olefins. The etherificationeffluent from the B reactor contains MTBE, TAME, unreacted methanol andC₄ + hydrocarbons and is passed 126 to a mid-portion of debutanizer Dfor separation into an overhead stream 128 comprising methanol and C₄ -hydrocarbons and a bottom product stream 130 of ether rich C₅ +gasoline.

While the instant invention has been described by specific examples andembodiments, there is no intent to limit the inventive concept except asset forth in the following claims.

We claim:
 1. A reactor system for production of methyl t-butyl ether andmethyl t-amyl ether in high yield, comprising:first reactor means forcontacting methanol and a feedstream comprising C₅ hydrocarbon rich inisoamylene with acidic etherification catalyst in a first etherificationzone under isoamylene etherification conditions; first fractionationmeans for separating effluent from said first etherification zone toproduce a stream comprising unreacted methanol and C₅ hydrocarbon and aproduct stream comprising methyl t-amyl ether; second reactor means forcontacting said unreacted methanol and C₅ hydrocarbon stream, freshmethanol and a first C₄ feedstream comprising C₄ hydrocarbon rich inisobutylene into a second etherification zone under iso-olefinsetherification conditions in contact with acidic etherificationcatalyst; second fractionation means for separating effluent from saidsecond etherification zone to produce a second C₄ stream comprisingunreacted methanol and C₄ hydrocarbons and a product stream comprisingmethyl t-butyl ether and a product stream rich in methyl t-amyl ether;and means for introducing said second C₄ stream to said firstfractionation means to enhance the separation of unreacted methanol. 2.The reactor system of claim 1 including means for introducing at least aportion of said first C₄ feedstream to a top portion of said firstfractionation means as a reflux stream.
 3. A multistage continuousreactor system for production of high octane gasoline containingasymmetrical tertiary alkyl ethers, comprising:first reactor means forcontacting C₅ + hydrocarbon feedstream rich in isoamylene and an alkanolstream with acidic catalyst in a first etherification zone undertertiary olefin etherification conditions; first fractionator means forseparating first reactor effluent in conjunction with added C₄ +hydrocarbon stream, whereby a fractionator bottom stream comprisingalkyl tertiary amyl ether is produced and an overhead stream comprisingunreacted alkanol and C₄ + hydrocarbon; second reactor means includingmeans for introducing said overhead stream and fresh alkanol into secondetherification zone containing acidic catalyst under tertiary olefinetherification conditions to produce an effluent stream comprising alkyltertiary butyl ether, alkyl tertiary amyl ether, unreacted alkanol andC₄ + hydrocarbon; second fractionator means for separating secondreactor effluent to produce a stream comprising c₅ + ether rich highoctane gasoline and a stream containing unreacted alkanol and C₄ -hydrocarbon; and extraction contact means operatively connected betweensaid first reactor means and said first fractionator means for washingsaid first reactor effluent with water to remove water solublecomponents therefrom.